Biaxially oriented films made from conventional petrochemical-based polymers, such as biaxially oriented polypropylene (BOPP) films and biaxially oriented polyethylene terephthalate (BOPET) films have been used for packaging, decoration, and label applications, and often perform multiple functions. For example, in lamination they may provide printability, transparent or matte appearance, or slip properties; they may provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties; and they may provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
In recent years, interest in “greener” packaging has been developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gases. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and may be biodegradable as well. Bio-based polymers such as polylactic acid—which is derived from corn starch and thus, can be considered to be derived from a renewable resource—is one of the more popular and commercially available materials for packaging film applications.
For bio-based polymer films to be fit-for-use for many packaging applications such as snack foods, it is desirable that the bio-based polymer films match as many of the attributes possible of films made from BOPP and BOPET, which are well-known for heat sealability, printability, controlled COF, metallizability, gas barrier properties, for example. However, biaxially oriented polylactic acid base polymer (BOPLA) films can suffer from poorer thermal stability, poorer moisture barrier properties, and weaker metal adhesion properties than current petroleum-based packaging films. To address some of these deficiencies, one could use petrochemical or non-biodegradable base components to compensate for the disadvantages of BOPLA films in the packaging laminate structure. For example, one could use polyethylene or polypropylene-based materials to apply heat sealability and high gas barrier properties to the packaging laminate structure by laminating them to a simple BOPLA outer film where the BOPLA film provides a printable surface but not the sealability or barrier.
However, recently the desire for “greener” packaging is requiring 100% compostable, bio-degradable or renewable design in the packaging film. In this configuration, the component using BOPLA film is expected to provide desirable performances without relying on the conventional petrochemical-based resins or films. Therefore, BOPLA films are required to be more functionalized in order to provide a cost-effective package that allows for viable commercial success. The single sheet of BOPLA film needs to have multiple functionalities, such as high gas barrier and heat sealability. A particular concern for high barrier packaging is the poor moisture barrier property of BOPLA. The polar nature of PLA tends to be detrimental for achieving high moisture barrier properties. Without being bound by any theory, the thought is that water molecules—being polar themselves—may more easily migrate through a polar polymer film than a non-polar polymer film. In addition, plain crystalline BOPLA film presents difficulties in the down-stream conversion processes such as heat sealing, bag-forming etc, due to its high surface crystallinity resulting from the biaxial orientation process as well as due to the poor impact resistance, poor heat resistance, and poor formability of PLA polymer itself.
To provide a solution to the above requirement, various attempts have been devised. For example, U.S. Pat. No. 6,713,175 discloses a single layer BOPLA film having heat sealability with stretching stability and its polymer has a specific storage elastic modulus. U.S. Pat. No. 6,153,276 discloses a heat sealable PLA based polymer laminate having an amorphous PLA layer and a crystalline PLA base layer. WIPO Patent Application WO 2008/020726 discloses heat sealable BOPLA film having a B/A and B/A/B layer structure. Japanese Patent Application JP2004-358721A also describes a PLA film having a core layer with a heat sealable PLA skin layer that includes a slippery agent such as lubricant.
In addition, U.S. Pat. No. 7,368,160 discloses a co-extruded BOPLA film displaying improved winding and manufacturability and end-user performance. U.S. Pat. No. 6,600,008 discloses an aliphatic polyester film and gas barrier film having specific surface, thermal and orientation properties for packaging applications. U.S. Pat. No. 7,390,558 discloses a primer-coated biaxially oriented aliphatic polyester film having an amorphous skin layer and its inorganic deposited film having about 3 to 5 cc/m2/day of oxygen gas permeability. U.S. Patent Publication No. 2010/0040904 disclose a co-extrude BOPLA film metallized on a crystalline core PLA layer with metal seeding such as Cu or Ni.
The inventors seek to address the above issues of making high barrier metalized BOPLA films.